Compressor pistons historically were solid metal cylinders, structurally sound and with more than sufficient outer surface area, but inherently massive. Mass could be reduced only by axially shortening the piston, inevitably reducing the outer surface area. Since compressor pistons are typically driven by an inclined swash plate, the reciprocating forces applied to the pistons inevitably have non axial components that act to rock the piston about its axis within the cylinder bore. The outer surface area of the piston is needed to resist these rocking forces, so its outer surface area, ideally, would not be reduced too far from a complete cylinder.
The obvious first approach to maintaining piston outer surface area while reducing mass is to completely hollow out the piston body itself. Just as obviously, this cannot be done in a one piece design. That is, the lower end of a bottle can be integrally formed, but its lid cannot. Thus, myriad designs have been suggested in the prior art where the end cap of the piston, the lid of the bottle, in effect, are attached by numerous techniques. While these are undeniably low mass, with complete outer surface areas, a multi piece design, requiring an extra manufacturing step to join the multiple pieces, is inevitably higher cost than a one piece or unitary design.
The next iterations in the continuing quest to produce compressor pistons that were not solid and massive, but still unitary, were various “hollowed out” designs. That is, internal mass was removed, reducing mass and weight, but outer surface area was inherently removed as well by the process of “hollowing”, whether that process was forging, molding, or machining. These “hollowed out” designs produced many ultimate shapes, most of which were impractical and did not see ultimate production.
An early design in this area is seen in Japanese Patent 2924621, first published in 1995 as Laid Open Application 7-189900. Two ways of hollowing out the piston were proposed there. One hollowed out both sides of the piston up to, and stopping at, a solid web of material at the central plane of the piston. This created an I beam shape, in cross section, leaving outer surface area only at the top and bottom of the piston outer envelope, but none on the sides. The other embodiment hollowed one entire side, leaving a thin walled, C shaped shell on the other side, but with essentially no outer surface area left on the hollowed out side. Both designs had the advantage of being moldable or forgable, that is, formable by only two tools or dies that approach and part along a straight line. None of the internal surfaces, as seen in cross section, present any concavities or “under cuts” relative to the line of tool parting, which can be considered either a result of, or an enabler to, the manufacturing method. Neither design was particularly practical, since one removed too much sides surface area, and the other, while it left a good deal of surface area at least on the critical piston side, left no internal support for the thin, C shaped shell.
A design that followed soon after, disclosed in co assigned U.S. Pat. No. 5,630,353, incorporated herein by reference, was also a hollowed out shape, but with no central web, being hollow through and through, as viewed from the side. A reference frame for the outer surface of the piston was designated in FIG. 8 of the patent, arbitrary but convenient, which divided up the potential outer surface area or envelope of the piston into four basic quadrants or sections, with a central plane P arrayed on the 12 o'clock-six o'clock line. In that context, a radially inwardly facing quadrant I is centered at the 12 o'clock point, an opposed radially outwardly facing quadrant O defined is centered at the 6 o'clock point, and two opposed side quadrants S are centered at the 3 and 9 o'clock points respectively, each subtending 90 degrees of the total 360 degrees. Shorter cylinders F and B at the front and back of the envelope represent, in effect, the top and bottom of the bottle, while the other quadrants divide up the outer surface of the bottle. This reference frame for the piston is defined, most generally, relative only to an arbitrary central plane P of the piston itself. In terms just of how the piston shape is described, it is not necessary that the piston reference frame correspond to the reference frame of the cylinder block/compressor, that is, it isn't necessary that the central plane P of each piston also, if extended, contain the axis of the cylinder block/shaft. The terms “radially facing”, whether inwardly or outwardly, have to be understood, then, in the context of the reference frame of just the piston's center axis itself, considered alone. That is, the radial inward and outward directions might or might not correspond to the same directions relative to the cylinder block/shaft axis. It is convenient for ease of manufacturing the piston as a whole, however, that the central planes P of each piston do intersect the block/shaft axis, so that so that the radial directions “in” and “out” would match, and that is the convention used here.
This arbitrary reference frame, recreated here in FIG. 8 as well, conveniently demonstrates the various features and shortcomings of the myriad designs proposed in the published patent literature. For example, the first, “I beam” embodiment of Japanese Patent 2924621 has surface area on both quadrants I and O, but essentially no surface area on the side quadrants S, with a heavy web at the central plane P. The second, C shaped embodiment has surface area arrayed over I, O and one side quadrant S, but essentially none on the other side quadrant S, and with no central support for the thin walled shell. The design in U.S. Pat. No. 5,630,353 improved on both of these embodiments with a shape that provided surface area on I and O, no heavy web on the central plane P, but with special “sled runner” features 40 that put some surface area, at least, on both side quadrants S. The shape disclosed there was still moldable or formable by only two tool elements. However, a drawback of both this design and the first embodiment of Japanese Patent 2924621 is that the wall thickness radially inboard of the piston outer surface area is greater than is ideal. That is, as seen in a cross section normal to the piston length axis A (FIG. 7 of U.S. Pat. No. 5,630,353, for example), the wall section is lunate in shape, that is, it has a cross sectional area essentially bounded by the arc and chord of a circle, far thicker and heavier that a section consisting of two closely spaced and concentric arcs. But, the flat, chordal side of the wall section is what is inevitably left behind by the advancing and retreating forging die or casting mold. On the other hand, the C shaped cross section of the second embodiment of Japanese Patent 2924621 is far thinner, consisting basically of two closely spaced concentric arcs, but, as noted, it has almost no surface area on one side quadrant S, and almost no central internal support to the thin wall.
A plethora of patented designs subsequent to these two early disclosures have dealt with these various design constraints with varying degrees of success. U.S. Pat. No. 5,765,464 catalogs the various prior art hollow, or hollowed out, piston designs at that point, noting that one prior design in particular, shown in FIG. 3a, hollowed out the piston with two intersecting cavities, each of which primarily removed surface area from the I and O surface quadrants (as discussed relative to the instant FIG. 8 above). This was described as removing too much outer surface area, but did at least have the advantage of being a unitary, one piece design. The improvement touted by the patent itself, while having more outer surface area, is not a one piece design, needing a separate cap to close of the F end section shown in FIG. 8. A multi piece design is far less desirable than a one piece design.
A more recent patent, U.S. Pat. No. 6,324,960, discloses a variant of the I beam embodiment shown in Japanese patent 2924621 discussed above. As best seen in its FIG. 11A, the lunate, overly thick wall section has been machined out at 224 and 226, thinned out to more closely match the ideally thin, concentric arcs shape. However, this is achieved only at the cost of an additional machining step, done after the molding or forging process.
In conclusion, the piston art to date has failed to achieve an ideal combination of one piece, substantially hollow construction with a well distributed outer surface area that is internally well supported, but with minimal wall thickness behind the outer surface area, and which is also formed with a minimum of manufacturing steps.